The lion turtle is an ancient species; the only specimen known to be alive is known as the oldest living thing on the planet.
When Sozin's Comet returned in 100 ASC, there was only one known member of the species to be alive. However, paintings in Wan Shi Tong's Library depict a much smaller lion turtle without vegetation growing on its shell being followed by two others. This shows there were once more lion turtles than the one known that survived to the end of the Hundred Year War.
Most of the lion turtle's body is that of a turtle; however, it has the face and paws of a lion. It has webbed feet, and its finger joints are made of soft tissue that is the same as a real-life crab's joint cover. Its rheum is not sticky or soft, and its pupil is slightly blurry, as if it has a cataract. Lion turtles can attain a great age if they are not harmed, with the oldest one being the oldest known creature on Earth. If a lion turtle is allowed to age for many eras, it becomes as large as an island, and its shell becomes covered with plant life and certain animal life. At a younger age, the lion turtle is not much bigger than a flying bison, with no plant or animal life covering it.
A lion turtle appeared in the beginning of the unaired pilot as a statue behind Aang.
A giant turtle-like being with an island, or the entire world, growing from its shell is quite popular in several mythologies.
In Hindu mythology Vishnu's second avatar is a giant turtle (Kurma) who holds the entire world on its back. Moreover, Vishnu's fourth avatar is a half-man, half-lion (Narasimha) who helps a pious little boy (Prahlad) destroy an evil king (the Buddha too is considered an avatar of Vishnu). In that sense, the lion turtle can be a reference to a god-like or buddha-like figure explaining to Aang how to defeat the Fire Lord.